The invention relates in general to contactless radiation measurement to determine the temperature of an object independently of its emissivity as well as to a method of determining spectral emissivity of an object, and also to a device for carrying out the method.
The radiation measurement to determine the temperature and/or emissivity of a natural or artificial object can be performed in infrared and/or visible spectral range (when measuring glow temperatures, for example), measuring spectral radiance or spectral radiant intensity.
Contactless measurements of this kind are applicable in technological processes, where quality of processed products depends strongly on the maintenance of preset temperatures or temperature variations; for example, such requirements occur during firing of highly accurate ceramic parts, melting of metal alloys, or zone drawing of semiconductive materials for doping. In prior art temperature measurements of this kind, there is the difficulty that if the emissivity of the object under measurement is not known, only an apparent temperature and not the actual temperature of the object can be measured.
This is due to the fact that in radiation temperature measurement, the sum of two radiation components is measured, namely radiation emitted by the object according to its temperature and its emissivity .epsilon. and ambient radiation (corresponding to the temperature of environment) reflected from the object according to its reflectivity .rho..
Only with the knowledge of the emissivity of the object the two components can be separated one from the other. (It is assumed that radiation transmittance of the object is negligible.)
Therefore, the disadvantage of known methods and devices of this kind is the fact that they cannot measure the emissivity and, consequently, the emitted radiation component and the reflected radiation component cannot be separated.
Known are so called emission meters by means of which emissivity of materials for scientific applications can be measured in laboratories. These instruments, however, are unsuitable for use under real manufacturing conditions in processing or construction or building technology, for example. Such measurements under real conditions in practice are of particular interest inasmuch as they enable also to find out the effect of weathering and aging on the emissivity of objects.
In building industry and in solar technology the knowledge of emissivity of materials and construction substances is important also due to the fact that it enables to optimize the radiation balance of buildings or solar collectors to save energy. For instance, it is desirable that a solar collector receive as much radiation from sun, as possible while an apartment or office building should radiate a minimal amount of infrared radiation into environment.
Also in many application fields using remote sensing from airplanes or satelites, the knowledge of emissivity of materials and natural or artificial objects is an essential prerequisite for achieving quantitatively interpretable results. (For example in remote prospecting for mineral resources, in mineralogy, geology, agriculture and forestry, in protection of environment, city planning, climatology, classification and the like).
In conventional methods of contactless temperature measurements used in practice, there are employed radiometers or pyrometers which measure integrally radiation in a relatively broad spectral range between about 3.0 to 3.5 .mu.m or 8.0 to 14.0 .mu.m. From the radiance or radiant intensity integrated within the spectral range, only an apparent temperature of the object is determined by a calibrating measurement. Since the measuring and calibrating processes can be performed only rarely under identical geometrical conditions (distances), an error in the temperature determination would result, as it will be explained below in connection with FIGS. 1 and 2. Moreover, when the emissivity of an object is not known and differs from unity (.epsilon..noteq.1) the result becomes still more erroneous because without correction it would be correct only if the object would be a black body, having emissivity .epsilon.=1 just as the calibration standard.
In prior art methods for determining emissivity, there are used the so called emission meters, in which a sample or specimen of a material, or of an object to be measured is placed in a closed, temperature stabilized housing with black inner walls (i.e. emissivity of inner walls equals 1), and heated to a temperature which exceeds the temperature of the housing.
Provided that the temperatures of the housing and of the sample are known, (FIG. 2) the emissivity of the sample can be determined by means of two radiation measurements, namely of those of the heated sample and of the black inner wall of the housing. Moreover, it is necessary that the temperature distribution both on the sample and on the housing be homogenous. However, a homogenous temperature distribution on the housing cannot be guaranteed especially in the case of high temperatures of the sample, because the wall absorbs the sample radiation and heats up. An accurate temperature detection on the inner wall of the housing is extremely difficult. Due to the heating process, the temperature of the probe is in general non-homogenous.
Known is also a method of and a device for contactless measurement of infrared radiation temperature of a natural or artificial object in which the radiation measurement is made in two or more delimited spectral ranges in which the transmissivity of atmosphere equals at least approximately to unity (1). To determine in a single measuring cycle the temperature and/or emissivity of the object, a characteristic curve of spectral radiance is computed from the measured radiation intensities in different spectral ranges, by iterative computation by the aid of Planck radiation law, in such a way that the temperature is determined of a black body whose spectral radiance curve extends parallel to the measured intensities. The emissivity of the object can be determined from the ratio between the measured intensities and corresponding intensities of the computed black body (DE-OS3,115,887).
From the above considerations it can be deduced that prior art method and devices for contactless temperature measurement can provide an unambiguous temperature reading only after the knowledge of the emissivity of the object of measurement, and of the ambient radiation or ambient temperature. In addition, it is also necessary to correct effects of the atmosphere by detecting the transmissivity of the latter.
The disadvantage of the prior art method and devices for emissivity measurement is the fact that temperatures of the measured sample or probe and of the measuring housing cannot be determined from radiation measurements employed for the emissivity determination. Moreover, the temperature measurement of samples in a glowing condition (glowing melts) is either completely impossible or extremely inaccurate (due to missing information about emissivity) by means of pyrometers. In this case a strong heating of the measuring housing takes place producing a further measuring error. A further disadvantage of the known method and device for emissivity measurement is therefor to be seen in their unsuitability for measuring at high temperatures (of glowing melts, for example). To solve the before mentioned difficulties concerning the constant temperature and temperature distribution of the probe, expensive regulating systems for ceoling the housing are necessary which render the existing methods and devices too costly for practical use.